Near-Infrared Wavelength Tunable Light Source (2018-2019)
Calibration of optical detectors is important for the reliability of optical characterization methods and therefore, has significant applications in optics and metrology. We have developed a portable wavelength tunable light source to calibrate the spectral power responsivities of detectors in the infrared region.
This light source improves spectral responsivity calibrations of the detectors. The setup decreases the expanded uncertainty of calibrations to approximately 2.2% – 2.6% from 4% achieved earlier using monochromator-based source. The source comprises of a supercontinuum laser, a laser line tunable filter (LLTF), and an output coupling optics with a focusing lens and a cut-on filter. To automate this setup a motorized XY translation stage and a filter wheel was used. The setup operates over a spectral range of 800–2000 nm with a maximum power of approximately 0.7 mW and full-width-at-half-maximum of 3 nm at a wavelength of 1100 nm.
We have observed that the LLTF produces strong second and third harmonics in the visible wavelength range. Therefore, a cut-on filter with up to 99.9% attenuation of all the wavelengths below 800 nm was used for filtering the harmonics. The laser setup has an average power of up to 0.3 mW within a 3 nm band from 800 nm to 2000 nm. In order to avoid saturation of the detectors, an interchangeable neutral density filter with the optical density of 0.5 is used to attenuate the optical power of the beam. The setup produces a collimated uniform beam with a narrow diameter of approximately 2 mm.
Three photovoltaic detectors, of which two were based on indium gallium arsenide (InGaAs) and one based on germanium, were calibrated using the automated wavelength tunable light source setup against a pyroelectric radiometer [1]. The spectral responsivities of the photovoltaic detectors were compared to the older calibrations performed using a reference spectroradiometer at the Metrology Research Institute. Through careful analysis of the experiments and comparisons, it is found that this setup decreases the expanded uncertainty in detector calibrations. However, the measurement uncertainty at wavelengths below 820 nm and above 1600 nm are slightly higher. This is due to the limitation of the optimal working spectral range of LLTF i.e., 1000 nm – 2000 nm. The standard uncertainty also increases near the band gap edge of the photovoltaic detectors.
Contact persons: Kinza Maham, Petri Kärhä, Farshid Manoocheri
References
[1] Maham, K., Vaskuri, A., Manoocheri, F., Ikonen, E. “Calibration of Near-Infrared Detectors Using a Wavelength Tunable Light Source,” Opt Rev (2020). https://doi.org/10.1007/s10043-020-00586-9
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